When handling mineral wool products large or small amounts of dust are formed, which dust includes fibres, among other things. It is a general aim to minimize the amount of dust, including fibrous dust, to which working people are exposed, and the present invention is intended to solve this problem.
The invention is based on studies of the mechanisms which cause appearance of dust and also the mechanisms which can be used to prevent spreading of dust. These studies have shown that the air-borne dust which is produced when handling mineral wool mainly comprises thin, short mineral fibres. The studies also have shown that the air-borne fibres and other particles emanate from the surfaces of the mineral wool products.
There are three mechanisms which, separately or in combination, can keep a fibre in a mineral wool product and can prevent the fibre from becoming air-borne. Said mechanisms are:
binding PA0 fastening PA0 mechanical locking.
The binding is effected by means of a binding substance. The binder generally is a thermosetting resin which in the form of small drops is distributed in the mineral wool products. If such a binder drop sticks to a fibre said fibre is prevented from becoming air-borne.
The fastening is a less heavy binding. The fastening can be effected by means of a dust binding oil, which in the form of a thin layer covers large portions of the fibre surfaces. Two fibres which are in contact with each other, and one fibre of which has an oil film on its surface, get fastened to each other and said fastening generally is sufficient to prevent the fibre from becoming air borne.
Investigations also have proved, however, that another type of fastening is of importance, namely an electrostatic fastening. The process of manufacturing mineral wool products includes a hardening stage in which the product and the binder for the product is heated to about 200.degree. C., whereby the binder is finally hardened. During the hardening stage the product also becomes completely dried, and since the hardening stage is combined with a heavy gas flow through the products the fibres may become electrostatically charged. In the subsequent cutting and packing etc. of the mineral wool product said electrostatic charge is maintained and fibres which have become charged tend to be maintained in the product. Upon storing and transporting, the product, however, becomes discharged and subsequently the fibres, which were once electrostatically fastened, then can become air borne.
The third mechanism for keeping the fibres in the mineral wool mass is the mechanical one. A sufficiently long fibre will always be in contact with a large number of other fibres and said fibre will be kept in the product solely by means of friction and will be prevented from becoming air borne.
The investigations also have shown that some fibres are so loosely fastened in the product, or not fastened at all, that they can easily be sucked off the product in that the product is moved passed a suction nozzle or a suction slot having a sufficient suction capacity. Other fibres and particles are so strongly fastened to the product that they cannot normally be removed from the product. Between said two groups of fibres and particles there is a group of fibres and particles which cannot be easily sucked off the product but which can still become air borne, in particular after the electrostatic force has disappeared.
Mineral wool products having a reasonable density, for instance rock wool products having a density of less than 50 kg/m.sup.3 are compressible to a substantial degree. Depending on the low density the porosity of the product is very high, generally more than 95%. Therefore, if such a product is compressed to about half its original thickness, a volume of air corresponding to about half the original volume of the mineral wool body must be pressed out of the product. It has been shown that the air pressed out of the mineral wool body can release and bring a substantial part of the loose fibres at the surface out of the product if the said compressing is made with a sufficient speed. This is the basis of the present invention.